So I switched a RV790 core from an HD 4890 onto a 4850m hoping it would work (stock core is a fully enabled RV770). I got the core on well, but I read full core and memory shorts with a multimeter, meaning BGA incompatibility. I'm thinking that the issue may just be unused GDDR5 connections on the PCB are grounded, which makes the cores with the extra GDDR5 lines routed shorted and incompatible. If this is the case, I could just leave those solder balls out if I knew where they were. If anyone has any access to RV770 (specifically GDDR3 versions, but 5 would be helpful too) and RV790 bga documentation, that would be great.

Below is a pic of the 4850m with the HD 4890 core. If a 4890 core can't work it would be great if a 4870 core at least could. The mobile cards got the worst of the worst binning chips and even a decent 4870 core would be a 20% upgrade...

It could be just random shorts, but I have doubts on that. Usually when I screw up a bga mount the die is slightly tilted on the pcb, but in this case it is level on all 4 sides. Both memory and core are shorted, while with a random short or 2 I would expect just one to be. This summer I'll have time to pull the core back off and check for shorts and redo it if I find any. Datasheets would be nice just to check if this is a complete waste of time or not. Right now I am operating under the assumption that cores that went onto gddr5 cards can also run on gddr3 cards, which is a big assumption.

Another concern is the rv790 core does have an extra ring of solder connections around the inner part of the package on the reverse side of the die. Niether gddr5 or gddr3 rv770 card cores have this. I'm thinking these are just power and ground for the extra die size and are not really needed, but maybe connections were reworked.

I'm also banking on the rv770 vbios to be able to run a rv790 core... yeah lots of reasons for this upgrade not to work, but it seems like it has a chance. I did throw a 9600xt core onto a mobility 9600 and it did work, despite the 9600xt core not being on the same process node as the original core.

OK, so I removed the solder balls (63/37) with a heat gun and found that I did a crappy job removing the original unleaded balls off the core, particularly around the outside where it shorted. I don't want to heat the core with a heat gun enough to melt off unleaded as that can kill the core, so now its time to wait for a desoldering braid to finish the job.

Btw, if you have time would be nice to see your rework setup and show that here on forums. It may benefit somebody who have enough wish to try something similar too.

Keep it up!

Lol its all a hackjob with no legit rework station. For cores I use a toaster oven with a thermocouple. The toaster is pretty bad at regulating the temperature on its own, so I set it high and crack the door open as the thermocouple says to actually regulate the temperature. This method actually leads to very accurate temperature regulation and I have not killed a single core while I've been doing it. The toaster works with mobile cards as they don't have big caps, but on desktop cards heating the whole card blows out the big through-board caps. I did rescue a 580 doing this once, and after after replacing a few of the caps it worked as good as new.

Memory is more resilient so I can use my heatgun and IR thermometor on them, but cores are too heat sensitive for a heat gun.

25' of desoldering braid is on the way so I shouldn't run out for a while. The card is already clean and just a little touch-up work is needed on the core and its good to attempt #2.

Unfortunately after this I have another project idea for my 980m... potential for major monetary damage but also major payoff (I think I figured out how Nvidia does laser cuts).

So I was really careful on the remount this time and everything went well... still shorts on both memory and core.

I'm not sure if the first try really did have shorts or not. It did after I pulled it, but those may have been caused by the removal process.

Prior to this attempt I did confirm that the extra ring of solder pads were all power and ground, and they were linked to the inner pads that I did have, making them unecessary.

Its probably just a few extra connections for gddr5 that are grounded on the pcb screwing everything up, but I have no practical way of figuring out what those are. I could probably find them by pulling all the memory chips of the 4890 pcb and check the memory connections vs the core connections... but this is not worth that kind of time sink.

Does anyone have an old 4850 with a golden core? At least I could put that on and get around 15% improvement.The ebay resellers even say that works.